Integrated circuit protection during high-current ESD testing

ABSTRACT

A method of protecting devices within an integrated circuit during electro-static discharge (ESD) testing using an ESD test system is provided. The method includes applying a direct current (DC) bias voltage to an input of at least one device of the integrated circuit and applying an ESD simulated signal to at least one other input of the integrated circuit. The applied ESD simulated signal is conducted along a first current path to a first ground, while a low-current signal associated with the at least one device is conducted along a second current path to the second ground. The DC bias voltage is maintained between the input of the at least one device and the second ground at a substantially constant value in response to a signal variation on the second ground that results from the applied ESD simulated signal.

BACKGROUND

a. Field of Invention

The present invention relates generally to the electrostatic discharge(ESD) protection of integrated circuits, and more particularly, toproviding protection during ESD testing of integrated circuits.

b. Background of Invention

An electrostatic discharge (ESD) event may cause spurious or unwantedcurrent spikes to flow through an integrated circuit. As a result, oneor more devices within the integrated circuit may become damaged basedon electrostatically discharged high current values (e.g., 1-10 A)causing dielectric (e.g., gate oxide breakdown) and/or metal (e.g., wireburn out) failure. As integrated circuit devices shrink in size, theymay become more susceptible to ESD damage.

Integrated circuit manufacturers or other entities may engage in ESDtesting in order to determine how resilient their integrated circuitdevices are to ESD discharge events. Accordingly, it may be advantageousto protect the various devices within an integrated circuit during theESD testing process.

SUMMARY

According to at least one exemplary embodiment, a method of protectingdevices within an integrated circuit during electro-static discharge(ESD) testing using an ESD test system is provided. The method mayinclude applying a direct current (DC) bias voltage to an input of atleast one device of the integrated circuit and applying an ESD simulatedsignal to one other input of the integrated circuit. The applied ESDsimulated signal is conducted along a first current path to a firstground, while a low-current signal associated with the at least onedevice is conducted along a second current path to the second ground.The DC bias voltage is maintained between the input of the at least onedevice and the second ground at a substantially constant value inresponse to a signal variation on the second ground that results fromthe applied ESD simulated signal.

According another exemplary embodiment, an electro-static discharge(ESD) test system operable to test the ESD capabilities of an integratedcircuit is provided. The system may include a floating groundconnection, a first plurality of switch devices, and a first pluralityof contacts each operably coupled to the floating ground via acorresponding one of the first plurality of switch devices. The systemmay further include a second plurality of switch devices and a pluralityof charge storage devices each having a first terminal and a secondterminal, where the first terminal of each of the plurality of chargestorage devices is operably coupled to the floating ground connectionvia a corresponding one of the second plurality of switch devices. Alsoprovided is a second plurality of contacts that are each both operablycoupled to one of the devices within the integrated circuit and coupledto the second terminal of a corresponding one of the plurality of chargestorage devices. Each of the first plurality of contacts is coupled to acorresponding one of the second plurality of contacts and operable toprovide a connection to the floating ground via a corresponding one ofthe first plurality of switch devices.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

FIG. 1 is circuit block diagram of an electro-static discharge (ESD)test system according to known embodiments;

FIG. 2 is a schematic diagram of an electro-static discharge (ESD) testsystem according to known embodiments;

FIG. 3 is a schematic diagram of an electro-static discharge (ESD) testsystem that compensates for the occurrence of unwanted grounddisturbance signals during ESD testing, according to one embodiment;

FIG. 4 is a schematic diagram of an electro-static discharge (ESD) testsystem that compensates for the occurrence of unwanted grounddisturbance signals during ESD testing, according to another embodiment;

FIG. 5 illustrates experimental results associated with the operation ofthe ESD test system of the embodiment of FIG. 4;

FIG. 6 is schematic diagram of an automated electro-static discharge(ESD) test system according to one embodiment; and

FIG. 7 is an exemplary operational flow diagram associated with theautomated test system embodiment of FIG. 6.

The drawings are not necessarily to scale. The drawings are merelyschematic representations, not intended to portray specific parametersof the invention. The drawings are intended to depict only typicalembodiments of the invention. In the drawings, like numbering representslike elements.

DETAILED DESCRIPTION

According to exemplary embodiments described herein, one or moretechniques may be utilized in order to protect an integrated circuitunder test during ESD testing. ESD testing may include the applicationof ESD test waveforms to a location (e.g., a zap pad) on an integratedcircuit device under test, and monitoring (e.g., sense/voltage pads) theoccurrence of any device failures based on the resulting test waveforms.An ESD current test waveform may include a Human Body Model (HBM)waveform, which simulates a charged human body transferring itselectrostatic charge to an ESD sensitive device (e.g., an integratedcircuit). Another ESD current waveform may include a Machine Model (MM)waveform that simulates electrostatic charge discharged from a machineor tool. A Charged Device Model (CDM) waveform is yet another form ofsimulated ESD test signal that may be utilized during the ESD testing ofintegrated circuit devices. According to the exemplary embodimentsdescribed below, a Transmission Line Pulse (TLP) generator may beemployed to provide ESD waveforms that are similar to HBMcharacteristics. A TLP test system may couple the generated ESDwaveforms to a test location on an integrated circuit via a contact suchas a zap pad, while one or more monitoring contacts such assense/voltage pads may be used for both signal monitoring and theapplication of external DC bias voltages to the various components(e.g., gate of an NMOS transistor device) within the integrated circuit.

FIG. 1 refers to a circuit block diagram of an electro-static discharge(ESD) test system 100 according to a known embodiment. The test system100 may include one or more external DC bias input 102 pads that areoperable to provide DC voltages to one or more devices under test 104(e.g., gate of a MOSFET device) within an integrated circuit 106. Theapplication of DC bias voltages to the various device(s) under test 104provides for the characterization of each device under different ESDconditions and applied DC bias voltages. For example, a device orcircuit under test may exhibit different tolerances to applied ESD testwaveforms based on the application of different DC bias voltages. Testsystem 100 may also include one or more input pads 108 for applying anESD waveform generated by, for example, a TLP test system (not shown) tothe one or more devices 104 and/or an ESD protection circuit 110 withinthe integrated circuit 106.

In operation, upon application of an ESD test waveform 112 to input pad108, the ESD protection circuit 110 provides a discharge path alongpaths P1 and P2 to ground connection 114 of the integrated circuit 106,which is denoted as a silicon or local ground LG. The local ground LG ofthe integrated circuit 106 is also connected to the ground connection116 of test system 100, which is denoted as system ground SG. Asillustrated, a wiring resistance 118 exists in connection 115 betweenlocal ground LG and system ground SG. Although resistance values inconductors may be relatively small, they are capable of exhibitingvarious parasitic effects, an example of which is illustrated anddescribed. Thus, as ESD test waveform 112 applied to input pad 108 isdischarged along paths P1 and P2 to local ground connection LG, andfurther discharged through path P3 via connection 115 and wiringresistance 118 to system ground SG, a transient ground disturbancevoltage 120 is generated at the local ground LG. The transient grounddisturbance voltage 120 is produced as a result of the high currentassociated with the ESD test waveform 112 passing through the wiringresistance 118.

For example, an 8 A current associated with an ESD test waveform passingthough a 0.5Ω wiring resistance generates a voltage disturbance of 4V atthe local ground LG. This causes, for example, a DC biased (e.g., 1V DC)gate input of a device such as an NFET transistor (not shown) to haveits ground raised by the 4V transient ground disturbance, which in turnreverse biases the NFET oxide interface by 3V (i.e., 1V−4V=−3V).Excessive forward or reverse bias voltages that are generated in thismanner may be sufficient to cause permanent damage to devices undertest. Such ground voltage disturbances and corresponding exemplarycompensatory measures are illustrated and described in relation to theembodiments of FIG. 2-4.

FIG. 2 refers to a schematic diagram of an electro-static discharge(ESD) test system 200 according to a known embodiment. FIG. 2illustrates an example of how a generated transient ground disturbancesignal 202 may cause damage to a device under test (DUT) 204 that isassociated with an integrated circuit 205. Although for brevity the DUTis identified as device 204, according to one or more other exampleconfigurations, a DUT may include the entire, or any portion of,integrated circuit 205, whereby the integrated circuit 205 incorporatesan ESD protection circuit 222 and protected circuit 225. The testportion of the system 200 may include one or more external DC voltagesources (not shown) that may provide DC bias voltages to the deviceunder test 204 via, for example, a coaxial cable 206. The test portionof the system may further include a test system ground plane 208 and aTLP pulse generator (not shown) that is operable to generate simulatedESD signals 210 via, for example, a coaxial cable 212 to a test input(e.g., zap pad) 214 of the integrated circuit 205. The tested integratedcircuit 205 of the system 200 may include an ESD protection device 222having a diode network or diode-pair 218 and 220, and a power clampcircuit 224, such that diode 218 operably conducts a positive ESD pulseto the power clamp circuit 224. Upon the power clamp circuit 224receiving the positive ESD pulse, the power clamp circuit 224 conductsthe positive ESD pulse to the integrated circuit's ground, which isdenoted as silicon ground or Local Ground LG, and then to system groundSG through pad 280. Similarly, diode 220 operably conducts a negativeESD pulse to the power clamp circuit 224, where the power clamp circuit224 also conducts the negative ESD pulse via the integrated circuit'sground LG to system ground SG.

The nFET device (i.e., an example DUT) 204 is DC biased via pad 228 atits gate input Q. In order to facilitate the DC biasing of the DUT 204,an external DC voltage source (not shown) is connected to the pad 228via, for example, a coaxial cable 206. The center conductor 234 of thecoaxial cable 206 is connected to pad 228, while the outer conductor(i.e., shield) 236 is connected to the system ground SG. The TLP pulsegenerator (not shown) generates the ESD signal 210, which is provided totest pad 214 via coaxial cable 212. The center conductor 240 of thecoaxial cable 212 is connected to pad 214, while the outer conductor(i.e., shield) 242 is also connected to system ground SG.

In operation, the TLP pulse generator (not shown) generates the ESDpulse signal 210, which is applied to the test pad 214 along signal pathH1. Upon receiving the ESD pulse signal 210 at pad 214, diode 218 isforward biased and conducts the ESD pulse signal 210 along path H2 topaths H3 and H4. As the ESD pulse signal is received by the power clampcircuit 224 along path H4, the ESD pulse signal is further conductedthrough the power clamp circuit 224 to local ground LG and pad 280 alongpath H5. The ESD pulse signal is then conducted along path H6 to systemground SG via wiring connection 246. The current paths (i.e., H1-H6)propagated by the ESD pulse signal are the high-current path. Since theESD pulse signal is a high-current signal, as the high-current ESD pulsesignal passes through the wiring connection 246, a voltage drop occursacross the wiring connection 246 as a result of a finite wiringresistance associated with the connection 246. Thus, based on theoccurrence of the voltage drop across the wiring connection 246, thetransient ground disturbance signal 202 is generated between the localLG and system SG grounds, as denoted by voltage V_(P), located at pointP.

As the DC bias voltage is applied to input pad 228 via cable 206, thegate of the nFET device 204 is DC biased relative to the system groundSG, as denoted by V_(Q), located at point Q. As the gate of the nFETdevice under test 204 turns on as a result of the applied DC bias, alow-current path is traversed along paths L1, L2, and L3 to the localground LG. The low current signal then passes along path L4 on the localground LG and shares a portion of the high-current path (i.e., H6) atpath L5.

Under normal operating conditions, since the system ground SG of thetest system and the local ground LG are connected, the bias voltagebetween the gate of DUT 204 and local ground LG, as denoted by V_(bias),is equivalent to the applied DC bias voltage denoted by V_(Q). However,when the transient ground disturbance signal 202 is generated betweenthe local LG and system SG grounds, the resultant voltage (V_(p)) raisesthe voltage level of the local ground LG relative to the system SGground.

The actual bias voltage, as seen by the gate of the device 204, is givenby:V _(bias) =V _(Q) −V _(p)  Equation (1)

For example, if the transient ground disturbance signal 202 generates avoltage of V_(p)=+4V and the applied DC bias voltage has a value ofV_(Q)=+1 v, according to Equation (1) above, the DC bias voltage willhave a value of V_(bias)=−3 v. This negative bias value of −3 v DC maycause permanent damage (e.g., gate oxide breakdown) to the DUT 204 whenevaluated under ESD test conditions based on the raised voltage (e.g.,V_(p)=+4 v) that occurs on the local ground LG of the integrated circuit205.

In accordance with one embodiment, FIG. 3 refers to a schematic diagramof an electro-static discharge (ESD) test system 300 that compensatesfor the occurrence of unwanted ground disturbance signals that may occurduring ESD testing. The electro-static discharge (ESD) test system 300differs from test system 200 (FIG. 2) in that a charge storage devicesuch as a capacitor 303 is coupled between the DC bias input to DUT 304and the local ground LG (also referred to as silicon ground) ofintegrated circuit 305. Also, an external DC voltage source (not shown)is connected to test pad 328 via, for example, a modified coaxial cable330, which includes cable sections 331A, 331B, and 331C. The centerconductor 350 of cable section 331A is connected to test pad 328 viacenter conductor 360 of cable section 331B. The outer conductor 352(i.e., shield) of cable section 331A is broken out and connected to thecenter conductor 354 of cable section 331C, whereby center conductor 354is coupled to the local ground LG of circuit 325. Outer conductors 358and 356 of cable sections 331B and 331C, respectively, are connected tothe system ground SG. Thus, the DC voltage source is operable to providea DC voltage to the gate of the DUT 304 via pad 328 by connecting the DCsource voltage output to center conductor 360 of cable section 331B andconnecting the DC source ground to center conductor 354 of cable section331C.

FIG. 3 shows an example of how a generated transient ground disturbancesignal 302 that may cause damage to device 304 of circuit 325 iscompensated or prevented. The test portion of the system 300 may includeone or more external DC voltage sources (not shown) that may provide DCbias voltages to the device under test 304 via, for example, themodified coaxial cable 330. The test portion of the system may furtherinclude a test system ground plane 308 and a TLP pulse generator (notshown) that is operable to generate simulated ESD signals 310 via, forexample, a coaxial cable 312 to a test input (e.g., zap pad) 314 of theintegrated circuit 305. The tested integrated circuit 305 of the system300 may include an ESD protection device 322 having a diode network ordiode-pair 318 and 320, and a power clamp circuit 324, such that diode318 operably conducts a positive ESD pulse to the power clamp circuit324. Upon the power clamp circuit 324 receiving the positive ESD pulse,the power clamp circuit 324 conducts the positive ESD pulse to theintegrated circuit's ground LG. The integrated circuit 305 of the system300 may also include one or more devices and circuits 325 (i.e.,including DUT 304) within the integrated circuit 305, which are testedfor evaluating their resilience under simulated ESD conditions. Althoughfor brevity the DUT is identified as device 304, according to one ormore other example configurations, a DUT may include the entire, or anyportion of, integrated circuit 305, whereby the integrated circuit 305incorporates an ESD protection circuit 322 and protected circuit 325.

The nFET device 304 is DC biased via pad 328 at its gate input Q usingthe external DC voltage source and cable 330 in the manner explainedabove. The TLP pulse generator (not shown) generates the ESD signal 310,which is provided to test pad 314 via coaxial cable 312. The centerconductor 340 of the coaxial cable 312 is connected to pad 314, whilethe outer conductor (i.e., shield) 342 is coupled to system ground SG.

In operation, the TLP pulse generator (not shown) generates the ESDpulse signal 310, which is applied to the test pad 314 along signal pathH1. Upon receiving the ESD pulse signal 310 at pad 314, diode 318 isforward biased and conducts the ESD pulse signal 310 along path H2 topaths H3 and H4. As the ESD pulse signal is received by the power clampcircuit 324 along path H4, the ESD pulse signal is further conductedthrough the power clamp circuit 324 to local ground LG and pad 380 alongpath H5. The ESD pulse signal is then conducted along path H6 to systemground SG via wiring connection 346. The current paths (i.e., H1-H6)propagated by the ESD pulse signal are the high-current path. Since theESD pulse signal 310 is a high-current signal, as the high-current ESDpulse signal passes through the wiring connection 346, as previouslydescribed, a voltage drop occurs across the wiring connection 346 as aresult of a finite wiring resistance associated with the connection 346.Thus, based on the occurrence of the voltage drop across the wiringconnection 346, the transient ground disturbance signal 302 is generatedbetween the local LG and system SG grounds, as denoted by voltage V_(p),located at point P.

As the DC bias voltage is applied to input pad 328 via cable 330, thegate of the nFET device 304 (i.e., an example DUT) is DC biased relativeto the local ground LG, as denoted by V_(Q), located at point Q. As thegate of the nFET device under test 304 turns on as a result of theapplied DC bias, a low-current path is traversed along paths L1, L2, andL3 to the local ground LG. In the current exemplary embodiment, the lowcurrent signal then passes along path L4 on the local ground LG back tothe voltage source ground such that the low-current path (i.e., L1-L4)is separate from the high-current path (i.e., H1-H6).

Under normal operating conditions, the bias voltage between the gate ofdevice 304 and local ground LG, as denoted by V_(bias), is equivalent tothe applied DC bias voltage denoted by V_(Q). Moreover, when thetransient ground disturbance signal 302 is generated between the localLG and system SG grounds, the resultant voltage (V_(p)) raises thevoltage level of the local ground LG relative to the system SG ground.However, due to the DC biasing being implemented with respect to thelocal ground LG, any voltage level increase (V_(p)) caused by thetransient ground disturbance signal 302 on the local ground LG, alsoraises the voltage output from the DC voltage source. This is due to theDC voltage value being referenced to the local ground LG, whichexperiences the transient voltage disturbances caused under ESD pulseconditions.

The actual bias voltage, as seen by the gate of device 304, is given by:V _(bias) =V _(Q) −V _(LG)  Equation (2)

Under normal operating conditions where there is no voltage disturbanceon the local ground LG, the local ground voltage with be V_(LG)=0 v.Thus:V _(bias) =V _(Q)  Equation (3)

However, the transient ground disturbance signal 302 on the local groundLG (i.e., when: V_(LG)=V_(p)) will also raise the ground reference ofthe DC voltage source, which in turn will raise the DC voltage at thedevice 302 gate, and, thus maintain the applied DC bias voltage(V_(bias)) at the gate of the device 304 at a constant value. This isillustrated below, where:V _(bias)=(V _(Q) +V _(p))−(V _(LG) =V _(p))  Equation (4)V _(bias) =V _(Q) +V _(p) −V _(p)  Equation (5)V _(bias) =V _(Q)  Equation (6)

As shown, the modified cable 330 connection generally maintains the DCbias at the device 304 input during biasing. However, the raised voltageon the local ground LG due to the transient signal 302 has to propagatealong cable sections 331C and 331A to the DC voltage source in order forthe DC voltage source to detect the change in ground reference voltage,and accordingly generate a corresponding change in the bias voltage thatis applied to the device 304 via cable sections 331A and 331B. Due tothe effect of this propagation delay, the gate of the device 304 maystill experience a momentary change in DC bias voltage based on theground disturbance voltage. In order to compensate for the propagationdelay effect, the capacitor device 303 is utilized. The capacitor 303acts as a charge storage device for DC signals applied to the device 304input (e.g., gate). Therefore, the applied bias voltage (V_(bias)) ismaintained at a constant value (i.e., V_(bias)) at the charged capacitorterminal A.

Alternatively, the capacitor 303 acts as a short circuit forhigh-frequency signals such as the transient ground disturbance signal302, which has a pulse period equivalent to that of the ESD pulsesignal. Consequently, as a result of the capacitor 303 acting as a shortcircuit with respect to the transient ground disturbance signal 302 onthe local ground LG, the transient ground disturbance signal 302 atterminal B of the capacitor 303 also appears at terminal A of thecapacitor. Thus, since both the gate of the device 304 and the localground LG see the same disturbance, the voltage difference between thegate of the DUT 304 and the local ground LG remains zero. Thus, thecapacitor 303 holds constant the DC bias voltage applied by the DUT 304via the voltage source, while eradicating the effect the transientsignal 302 caused by the ESD pulse 310 on the local ground LG.

The embodiment of FIG. 3 provides two compensatory mechanisms, where themodified cable 330 maintains the constant DC bias applied to the DUT 304for any DC or long period pulses that may occur on the local ground LG,while the capacitor 303 maintains the constant DC bias applied to theDUT 304 for any short transient pulses such as the occurrence oftransient disturbance signal 302 on the local ground LG. This dualcompensatory mechanism is provided as a result of the separation betweenthe high-current path (i.e., H1-H6) and the low-current path (i.e.,L1-L4).

In accordance with another embodiment, FIG. 4 refers to a schematicdiagram of another electro-static discharge (ESD) test system 400 thatcompensates for the occurrence of unwanted ground disturbance signalsthat may occur during ESD testing. The electro-static discharge (ESD)test system 400 differs from test system 200 (FIG. 2) in that a chargestorage device such as a capacitor 403 is coupled between the DC biasinput to device 404 and the local ground LG (also referred to as siliconground) of integrated circuit 405. Although for brevity the DUT isidentified as device 404, according to one or more other exampleconfigurations, a DUT may include the entire, or any portion of,integrated circuit 405, whereby the integrated circuit 405 incorporatesan ESD protection circuit 422 and protected circuit 425. The testportion of the system 400 may include one or more external DC voltagesources (not shown) that may provide DC bias voltages to the device 404via, for example, a coaxial cable 406. The test portion of the systemmay further include a test system ground plane 408 and a TLP pulsegenerator (not shown) that is operable to generate simulated ESD signals410 via, for example, a coaxial cable 412 to a test input (e.g., zappad) 414 of the integrated circuit 405. As previously described, thetested integrated circuit 405 of the system 400 includes an ESDprotection device 422 having a diode network or diode-pair 418 and 420,and a power clamp circuit 424, such that diode 418 operably conducts apositive ESD pulse to the power clamp circuit 424. Upon the power clampcircuit 424 receiving the positive ESD pulse, the power clamp circuit424 conducts the positive ESD pulse to the integrated circuit's ground,which is denoted as Local Ground LG, and then to system ground SGthrough pad 480. Similarly, diode 420 operably conducts a negative ESDpulse to the power clamp circuit 424, where the power clamp circuit 424also conducts the negative ESD pulse via the integrated circuit's groundLG to system ground SG.

The nFET device 404 (i.e., an example DUT) is DC biased via pad 428 atits gate input Q. In order to facilitate the DC biasing of the DUT 404,an external DC voltage source (not shown) is connected to the pad 428via, for example, a coaxial cable 403. The center conductor 434 of thecoaxial cable 403 is connected to pad 428, while the outer conductor(i.e., shield) 436 is connected to the system ground SG. The TLP pulsegenerator (not shown) generates the ESD signal 410, which is provided totest pad 414 via coaxial cable 412. The center conductor 440 of thecoaxial cable 412 is connected to pad 414, while the outer conductor(i.e., shield) 442 is also connected to system ground SG.

In operation, the TLP pulse generator (not shown) generates the ESDpulse signal 410, which is applied to the test pad 414 along signal pathH1. Upon receiving the ESD pulse signal 410 at pad 414, diode 418 isforward biased and conducts the ESD pulse signal 410 along path H2 topaths H3 and H4. As the ESD pulse signal is received by the power clampcircuit 424 along path H4, the ESD pulse signal is further conductedthrough the power clamp circuit 424 to local ground LG and pad 480 alongpath H5. The ESD pulse signal is then conducted along path H6 to systemground SG via wiring connection 446. The current paths (i.e., H1-H6)propagated by the ESD pulse signal are the high-current path. Since theESD pulse signal is a high-current signal, as the high-current ESD pulsesignal passes through the wiring connection 446, a voltage drop occursacross the wiring connection 446 as a result of a finite wiringresistance associated with the connection 446. Thus, based on theoccurrence of the voltage drop across the wiring connection 446, thetransient ground disturbance signal 402 is generated between the localLG and system SG grounds, as denoted by voltage V_(P), located at pointP.

As the DC bias voltage is applied to input pad 428 via cable 406, thegate of the nFET device 404 is DC biased relative to the system groundSG, as denoted by V_(Q), located at point Q. As the gate of the nFETdevice under test 404 turns on as a result of the applied DC bias, alow-current path is traversed along paths L1, L2, and L3 to the localground LG. The low current signal then passes along path L4 on the localground LG and shares a portion of the high-current path (i.e., H6) atpath L5.

Under normal operating conditions, since the system ground SG of thetest system and the local ground LG are connected, the bias voltagebetween the gate of DUT 404 and local ground LG, as denoted by V_(bias),is equivalent to the applied DC bias voltage denoted by V_(Q). However,when the transient ground disturbance signal 402 is generated betweenthe local LG and system SG grounds, the resultant voltage (V_(p)) raisesthe voltage level of the local ground LG relative to the system SGground. This transient ground disturbance signal 402 generated betweenthe local LG and system SG ground is, however, compensated by thecapacitor 403, where capacitor 403 acts as a charge storage device forDC signals applied to the DUT 404 input (e.g., gate). Therefore, theapplied bias voltage (V_(bias)) of the DUT 404 is maintained at aconstant value (i.e., V_(bias)) at the charged capacitor terminal A.

Capacitor 403, however, acts as a short circuit for high-frequencysignals such as the generated transient ground disturbance signal 402,which has a pulse period equivalent to that of the ESD pulse signal.Consequently, as a result of the capacitor 403 acting as a short circuitwith respect to the transient ground disturbance signal 402 on the localground LG, the transient ground disturbance signal 402 at terminal B ofthe capacitor 403 also appears at terminal A of the capacitor. Thus,since both the gate of the device 404 and the local ground LG see thesame disturbance, the voltage difference between the gate of the device404 and the local ground LG remains zero. Thus, the capacitor 403 holdsconstant the DC bias voltage applied by the device 304 via the voltagesource, while eliminating the effect the transient signal 402 caused bythe ESD pulse 410 on the local ground LG.

The Capacitor 403 may include a value in the range of approximately1-100 nano-Farads (nF). While the embodiment of FIG. 4 compensates fortransient ESD generated pulses on the local ground LG of the integratedcircuit devices under test, it may not compensate for DC or long pulsesthat may occur on the local ground. Due to this embodiment (FIG. 4) nothaving a modified DC bias cable connection arrangement, as provided inthe embodiment of FIG. 3, it may be simpler to implement in an automatedESD test system (see FIG. 6). Capacitor 303 (FIG. 3) may also include avalue in the range of approximately 1-100 nano-Farads (nF). Bothcapacitors 303, 403 may contemplate the use of capacitors over the valueof 100 nF, although in some instances the use of larger capacitor valuesmay not provide any operational advantages. As previously described, thecapacitors act as: (1) a DC signal storage device operable to charge upto and hold the DC bias voltage that is applied to the DUT input (e.g.,gate) by the external voltage source; and (2) a short circuit totransient ground signals for maintaining the same potential differencebetween the two terminals of the capacitor, which are coupled betweenthe input (e.g., gate) of the DUT (e.g., MOSFET) and the local ground LGof the integrated circuit associated with the DUT.

The actual bias voltage, as seen by the gate of the device 404, is givenby:V _(bias) =V _(Q) −V _(LG)  Equation (7)

Under normal operating conditions where there is no voltage disturbanceon the local ground LG, the local ground voltage with be V_(LG)=0 v.Thus:V _(bias) =V _(Q)  Equation (8)

Under a ground disturbance scenario, however, due to capacitor 403, thetransient ground disturbance signal 402 on the local ground LG (i.e.,when: V_(LG)=V_(p)) will also appear at the DUT 402 gate as result ofthe capacitor 403 acting as a short circuit to this high frequencytransient ground disturbance signal 402. Thus, the applied DC biasvoltage (V_(bias)) between local ground LG and the gate of the DUT 404remains constant, and held at the provided DC bias value stored onterminal A by the capacitor 403. This is illustrated below, where:V _(bias)=(V _(Q) +V _(p))−(V _(LG) =V _(p))  Equation (9)V _(bias) =V _(Q) +V _(p) −V _(p)  Equation (10)V _(bias) =V _(Q) =V _(ExternalBias)  Equation (11)

FIG. 5 refers to measured results obtained using the exemplaryembodiment of FIG. 4. Graphs 502 refer to a measured voltage graph 504and measured current graph 506 when the device 404 gate voltage (V_(Q))is externally DC biased at +1 v. As illustrated, the measured voltagegraph 504 includes a simulated electro-static discharge (ESD) inputsignal 508 from a TLP pulse generator of about 2.7 v, which has a pulseperiod of approximately 100 nanoseconds (ns). The simulatedelectro-static discharge (ESD) input signal 508 generates acorresponding 100 ns current pulse 510 of approximately 2 A. The valueof the capacitor 403 (FIG. 4) was chosen as 10 nF during measurements.

As the simulated electro-static discharge (ESD) test input signal 508 isapplied, a ground disturbance signal 514 on local ground LG (FIG. 4)caused by the simulated electro-static discharge (ESD) test input signal508 is detected and measured. The measured ground disturbance signal 514is a transient 100 ns pulse that raises the ground potential from about0 v to approximately 1 v. Due to the capacitor 403 (FIG. 4) acting as ashort circuit with respect to the ground disturbance signal 514 beingreceived at terminal B, the voltage signal 512 at the gate of the device404 (FIG. 4) is also raised by the same voltage as the grounddisturbance signal 514. Thus, the net voltage potential between the gateof the device 404 (FIG. 4) and local ground LG (FIG. 4) remainsunchanged despite the occurrence of the ground disturbance signal 514.As denoted by ΔV on graph 504, the DC bias voltage between the localground LG and gate remains at the applied +1 v value during the grounddisturbance caused by ground signal 514.

The graphs depicted by 518 refer to a measured voltage graph 520 andmeasured current graph 522 when the DUT device 404 gate voltage (V_(Q))is DC biased at 0 v. As with the previous measurements, the measuredvoltage graph 520 includes a simulated ESD input signal 524 from a TLPpulse generator of about 2.7 v, which has a pulse period ofapproximately 100 nanoseconds (ns). The simulated ESD input signal 508generates a corresponding 100 ns current pulse 526 of approximately 2 A.The value of the capacitor 403 (FIG. 4) was also chosen as 10 nF.

As the simulated ESD test input signal 524 is applied, a grounddisturbance signal 530 on local ground LG (FIG. 4) caused by thesimulated ESD test input signal 524 is detected and measured. Themeasured ground disturbance signal 530 is also a transient 100 ns pulsethat raises the ground potential from about 0 v to approximately 1 v.Due to the capacitor 403 (FIG. 4) acting as a short circuit with respectto the ground disturbance signal 530 being received at terminal B, thevoltage signal 532 (i.e., DC biased at 0 v) at the gate of the device404 (FIG. 4) is also raised by the same voltage as the grounddisturbance signal 530. Thus, the net voltage potential between the gateof the device 404 (FIG. 4) and local ground LG (FIG. 4) remainsunchanged despite the occurrence of the ground disturbance signal 530.As denoted by ΔV on graph 520, the DC bias voltage between the localground LG and gate remains at the applied 0V value.

FIG. 6 refers to an automated electro-static discharge (ESD) test system600 according to one embodiment. Automated test system 600 may include afirst plurality or bank of switches 602, a plurality or bank ofcapacitors 604 associated with the first plurality of switches 602, afloating ground plane connection 606, and a second plurality or bank ofswitches 608 associated with the floating ground plane 606. Each of theplurality or bank of capacitors 604 (C1-C4) may compensate for transientpulses occurring on the local ground by utilizing the same operatingprinciples as those described in relation to the embodiment of FIG. 4.FIG. 6 illustrates an automated system that compensates for local groundtransient pulses for multiple DC biased devices or circuits undersimulated ESD test conditions.

The first plurality or bank of switches 602 and their correspondingplurality or bank of capacitors 604 constitute the force side of theautomated system, where an example integrated circuit 610 under testreceives input stimuli such as one or more external DC biasing andsimulated ESD signals from a TLP pulse generator. The force side of theautomated system 600 also includes a ground input 612 associated withthe ground of the TLP pulse generator providing the simulated ESD signalto the integrated circuit 610 being tested. Although for brevity theforce side is shown with four inputs, up to twenty-five or more forceside inputs and up to twenty-five or more monitor/sense side outputs maybe contemplated. Each of the plurality or bank of capacitors 604 (C1-C4)are connected between a corresponding one of the first plurality or bankof switches 602 (i.e., S1-S4) and inputs I/P1-I/P4. For example,capacitor C1 is connected between I/P1 and Switch S1, capacitor C2 isconnected between I/P2 and Switch S2, capacitor C3 is connected betweenI/P3 and Switch S3, and capacitor C4 is connected between I/P4 andSwitch S4.

In operation, for example, if an external bias is applied to I/P3,switch S3 is actuated to the closed position so that the capacitor C3couples the DC biased input (e.g., gate) of the device 616 (e.g.,MOSFET) under test to local ground 611 via the floating groundconnection 606. Similarly, for example, if an external bias is alsoapplied to I/P2, switch S2 is actuated to the closed position so thatthe capacitor C2 couples the DC biased input (e.g., gate) of the device617 (e.g., MOSFET) under test to local ground 611 via the floatingground connection 606. Designated ground input 612 (i.e., I/P4) isconnected to the floating ground 606 of the tester 600 by closing switchS5. Using the same described principles as those referred to in relationto FIG. 4, capacitors C2 and C3 compensate for and, thus, establish aconstant DC bias voltage at the inputs (e.g., gate) of devices 617 and616, respectively, during any occurrence of transient ground disturbancesignals that may be generated during the application of ESD pulsesduring ESD testing.

Also during operation, for example, a TLP pulse generator (not shown)may be connected between force input I/P1 and the designated groundinput I/P4. For both inputs I/P1 and I/P4, switches S1 and S4 are in theopen configuration since the capacitors (i.e., C1 and C4) are not beingused in connection with protecting the biased inputs of the one or moredevices (e.g., MOSFETs 616 and 617) under test within integrated circuit610. Further, in order to establish the high current ground path for theESD input I/P1, I/P4, as the designated ground connection, is connectedto the ground plane 606 by closing switch S5. During this exemplaryconfiguration of the test system 600, by closing switch S5, the testsystem ground associated with the TLP pulse generation system is coupledto the local or silicon ground 611 of the integrated circuit 610 via thefloating ground 606.

On the sense/monitoring side, the TLP pulse that is generated may bemonitored at O/P1 by maintaining switch S8 in the open position Also, onthe sense side, the DC biased input (e.g., gate) of the device 616(e.g., MOSFET) under test is monitored between O/P3 by maintainingswitch S6 in the open position, and O/P4 which establishes theconnection to the local ground 611. Further, the DC biased input (e.g.,gate) of device 617 (e.g., MOSFET) is also monitored between O/P2 bymaintaining switch S7 in the open position, and O/P4 which establishesthe connection to the local ground 611. As shown on the sense side,attenuation resistors R_(Atten) may be included to avoid damaging theinput circuitry of monitoring equipment such as oscilloscopes, frequencyanalyzers, etc. For example, an attenuation resistor 618 may be utilizedto attenuate the generated TLP pulse signal being monitored at O/P1.

The automated test system 600 embodiment depicted in FIG. 6 operates toprovide DUT protection under multiple DC bias conditions for both theembodiments 300, 400 depicted in FIGS. 3 and 4, respectively. The testsystem may be configured to test any integrated circuit by selectivelyassigning the test system inputs (e.g., bias input(s), TLP input, etc.)and accordingly configuring the position (e.g., open/closed) of theswitches.

FIG. 7 refers to an operational flow diagram 700 for configuring theswitch positions of automated test system 600. The operation of flowdiagram 700 may be implemented within a controller device as a set ofcomputational instructions (e.g., a computer program). As previouslyindicated, the first plurality or bank of switches 602, the secondplurality or bank of switches 608, and the plurality or bank ofcapacitors 604 associated with the first plurality of switches 602 maybe configured based on connecting the TLP pulse generator and theexternal DC bias voltage sources to several assigned inputs (e.g.,I/P1-I/PN). Also, the monitoring/sensing outputs (e.g., O/P1-O/PN) areassigned in accordance with the assigned inputs (e.g., I/P1-I/PN).

FIG. 7 is described with the aid of FIG. 6. For the TLP signal input,two inputs may be assigned (e.g., I/P1, I/P4), where the first input(e.g., I/P1) connects to the TLP pulse generator output and the secondinput (e.g., I/P4) connects to the TLP pulse generator's ground (702).The switch (e.g., S1) corresponding to the input (e.g., I/P1) thatconnects to the TLP pulse generator's output is actuated to the openposition so the capacitor (e.g., C1) is not connected to the floatingground 606 (704). The switch (e.g., S8) that corresponds to thesense/monitor side output (e.g., O/P1) for monitoring the TLP pulsereceived at the input (e.g., I/P1) is also actuated to the open positionin order for the sense/monitor side output to not be connected to thefloating ground 606 (706). The switch (e.g., S4) corresponding to theinput (e.g., I/P4) that connects to the TLP pulse generator ground isalso actuated to the open position so the capacitor (e.g., C4) is notconnected to the floating ground 606 (710).

For each external DC bias input, an input may be assigned (e.g., I/P2,I/P3), where the input (e.g., I/P2, I/P3) connects to the external biasvoltage generator's output (708). The switches (e.g., S2, S3)corresponding to the inputs (e.g., I/P2, I/P3) that connect to theexternal bias voltage generator outputs are actuated to the closedposition so the capacitors (e.g., C2, C3) are connected to the floatingground 606 (710) for compensating for ESD testing-based ground transientpulse signals. The switches (e.g., S6, S7) that correspond to thesense/monitor side outputs (e.g., O/P2, O/P3) for monitoring theexternal bias voltages received at the inputs (e.g., I/P2, I/P3) areactuated to the open positions in order for the sense/monitor sideoutputs (e.g., O/P2, O/P3) to not be connected to the floating ground606 (712). The switch (e.g., S5) that corresponds to the sense/monitorside output (e.g., O/P4), which is associated with the ground input(e.g., I/P4), is actuated to the closed position in order for thesense/monitor side output (e.g., O/P4) to be connected to the floatingground 606 (712) along with ground connection 611 and ground input 612.Thus, the actuation of S5 connects the low-current local groundassociated with device under test to the high-current system ground ofthe TLP pulse generator via the floating ground 606 of the automatedtest system 600.

The descriptions of the various embodiments of the present inventionhave been presented for purposes of illustration, but are not intendedto be exhaustive or limited to the embodiments disclosed. Manymodifications and variations will be apparent to those of ordinary skillin the art without departing from the scope and spirit of the describedembodiments. The terminology used herein was chosen to best explain theprinciples of the embodiments, the practical application or technicalimprovement over technologies found in the marketplace, or to enableothers of ordinary skill in the art to understand the embodimentsdisclosed herein.

What is claimed is:
 1. A method of protecting devices within anintegrated circuit during electro-static discharge (ESD) testing usingan ESD test system, the method comprising: applying a direct current(DC) bias voltage to an input of at least one device of the integratedcircuit; applying an ESD simulated signal to one other input of theintegrated circuit; conducting the applied ESD simulated signal along afirst current path to a first ground; conducting a low-current signalassociated with the at least one device along a second current path to asecond ground; and maintaining the DC bias voltage between the input ofthe at least one device and the second ground at a substantiallyconstant value in response to a signal variation on the second groundthat results from the applied ESD simulated signal; wherein applying thedirect current (DC) bias voltage comprises: connecting to the input ofthe at least one device a first connection having a center conductor andan outer conductor, the center conductor of the first connectiondelivering the DC bias voltage to the input of the at least one device;and connecting to the second ground a second connection having a centerconductor and an outer conductor, the center conductor of the secondconnection electrically coupling the outer conductor of the firstconnection to the second ground, and the outer conductor of the secondconnection connected to the first ground.
 2. The method of claim 1,wherein the maintaining of the DC bias voltage comprises capacitivelycoupling the DC bias voltage on the input of the at least one device tothe second ground using a charge storage device.
 3. The method of claim2, wherein the charge storage device comprises a capacitor having acapacitance in the range of about 1-100 Nanofarads (nF).
 4. The methodof claim 1, wherein the first current path associated with the ESDsimulated signal conducted to the first ground is separated from thesecond current path associated with the low-current signal conducted tothe second ground.
 5. The method of claim 1, wherein the firstconnection comprises a first coaxial cable, and wherein the secondconnection comprises a second coaxial cable.
 6. The method of claim 1,wherein the ESD simulated signal comprises a transmission line pulse(TLP) signal that simulates a human body model (HBM) electro-staticdischarge (ESD) discharge signal.
 7. The method of claim 1, wherein thefirst ground comprises a system ground associated with the ESD testsystem generating the ESD simulated signal.
 8. The method of claim 1,wherein the second ground comprises a silicon ground associated with theintegrated circuit.
 9. The method of claim 1, wherein the ESD simulatedsignal is applied to a test input on the integrated circuit.
 10. Themethod of claim 9, wherein the test input comprises an input associatedwith an electro-static discharge (ESD) protection device.
 11. The methodof claim 10, wherein the electro-static discharge (ESD) protectiondevice comprises: a diode network having a first diode and a seconddiode; and a power clamp circuit, wherein, the first diode forconducting a positive ESD pulse to the power clamp circuit, wherein uponthe power clamp circuit receiving the positive ESD pulse, the powerclamp circuit providing a conductive path to the first ground for thepositive ESD pulse; and wherein the second diode for conducting anegative ESD pulse to the power clamp circuit, wherein upon the powerclamp circuit receiving the negative ESD pulse, the power clamp circuitproviding a conductive path to the first ground for the negative ESDpulse.
 12. The method of claim 1, wherein the signal variation on thesecond ground is generated by a wiring resistance associated with anelectrical connection between the first ground and the second ground.